1. Field of the Invention
The present invention relates to a process and an apparatus for producing a planar body of an oxide single crystal.
2. Description of the Related Art
A single crystal of lithium potassium niobate and a single crystal of lithium potassium niobate-lithium potassium tantalate solid solution have been noted especially as single crystals for blue light second harmonic generation (SHG) devices for semiconductor lasers. The devices can emit even the ultraviolet lights having the wavelengths of down to 390 nm, thus the crystals can be suitable for wide applications such as optical disk memories, medicine and photochemical fields, and various optical measurements by using such short-wavelength lights. Since the above single crystals have a large electro-optic effect, they can be also applied to optical memory devices using their photo-refractive effect.
However, for the application of a second harmonic generation device, for example, even a small fluctuation in the composition of the single crystal may affect the wavelength of the second harmonic wave generated by the device. Therefore, a specific range of the composition required for said single crystals is severe, and the fluctuation in the composition should be suppressed in a narrow range. However, since the composition consists of as many as three or four components, growing a single crystal at a high rate is generally extremely difficult to achieve, while controlling the proportions of the components to be constant.
In addition, for optical applications, especially for the second harmonic wave generation, a laser beam having a short wavelength of, for example, about 400 nm needs to propagate in the single crystal at as high a power density as possible. Moreover, the photo deterioration has to be controlled to the minimum at the same time. In this way, since controlling the photo deterioration is essential, the single crystal has to possess good crystallinity for this purpose.
NGK Insulators, Ltd. suggested a xcexc pulling-down method for growing such a single crystal with a constant compositional proportions, for example, in JP-A-8-319191. In this method, a raw material, for example, comprising lithium potassium niobate is put into a platinum crucible and melted, and then the melt is pulled out downwardly gradually and continuously through a nozzle attached to the bottom of the crucible. The xcexc pulling-down method can grow a single crystal more rapidly than the CZ method or the TSSG method does. Moreover, the compositions of the melt and the grown single crystal can be controlled by growing the single crystal continuously while supplementing the raw materials for growing the single crystal to the raw material melting crucible.
However, there is still a limitation in using the xcexc pulling-down method to grow a good single crystal plate (a planar body of a single crystal) continuously at a high rate.
The present inventors tried to form a shoulder portion or shoulder portions by adjusting the temperature of the melt, the ambient temperature around a fiber, etc. when an oxide single crystal fiber (seed crystal) was firstly contacted to a melt and then the melt was pulled down. The width of the shoulder portion is gradually enlarged, and when it reaches the desired size, temperatures of such as a nozzle portion are slightly raised to stop the increase in width of the shoulder portion. After that, a planar body having a uniform width is continuously pulled down following a terminal end of the shoulder portion. According to this method, cracks are hard to progress from near a joint interface of the seed crystal and the planar body.
However, during further examination of this method, the following problems arouse. That is, although the width of the shoulder portion was being gradually enlarged as the shoulder portion of the planar body was being grown, the width of the shoulder portion stopped enlarging and could not reach a desired width (e.g. 80 mm) if the width exceeded a certain extent.
Moreover, in some cases, although the width of the shoulder portion reached a desired width, polycrystal regions, cracks and crystal deteriorations occurred in a central portion of the shoulder portion in some cases.
It is an object of the present invention to be able to grow a shoulder portion having a larger width and to prevent polycrystal regions, cracks and crystal deteriorations in the central portion of the planar body, when the planar body of the oxide single crystal is grown with the xcexc pulling-down method.
The present invention relates to a process for producing a planar body of an oxide single crystal, said process comprising the steps of melting a raw material of said oxide single crystal in a crucible, contacting a seed crystal to a melt of the raw material near an opening of a nozzle of the crucible, drawing said melt from said opening by pulling down the seed crystal, and growing said planar body, wherein the vertical temperature distribution of said nozzle against said drawing direction is controlled by supplying heat to the nozzle and/or removing heat from the nozzle.
The present invention also relates to an apparatus for producing a planar body of an oxide single crystal comprising a crucible for melting a raw material of said oxide single crystal and temperature controlling unit, said crucible having a nozzle provided with an opening and said temperature controlling unit controlling a vertical temperature distribution of said nozzle against said drawing direction by supplying heat to the nozzle and/or by removing heat from the nozzle, wherein a seed crystal is contacted to said melt and the oxide single crystal is grown by pulling down the seed crystal to draw said melt from said opening of the crucible.